In recent years, various devices have been provided with a so-called touch panel (a display panel provided with a position detection system). Along with this trend, development of touch panels has also been advanced (Patent Document 1, for example). When a position detection system provided in such touch panels employs a relatively simple optical detection system, the structures illustrated in partial cross-sectional views of FIG. 12 or FIG. 13 are often used.
For example, a position detection system “pm” of FIG. 12 includes an LED (Light Emitting Diode) group 122 in which a plurality of LEDs 123 are arranged, and a line sensor 125 in which light receiving elements 126 are arranged. The position detection system “pm” identifies the position of an object such as a finger in accordance with a shadow of the object positioned between the LEDs (light sources) 123 and the line sensor 125. In other words, the position of the object is identified by whether the line sensor 125 receives light or not (whether a semiconductor included in the light receiving elements 126 of the line sensor 125 generates a reverse bias current by photoelectric conversion or not).
For this reason, it is desirable that the LEDs 123 emit light of a wavelength in which the semiconductor (photoelectric conversion body) included in the line sensor 125 is most likely to generate a reverse bias current. However, if light of the wavelength is visible light, it may possibly be seen by a user. Accordingly, the light sources in the position detection system of Patent Document 1 emit infrared light, which is invisible light.